Vehicles such as automobiles may be equipped with an electric power steering apparatus, which can generate an assist torque for assisting a steering torque generated in a steering system during operation of a steering wheel by the driver. By thus generating the assist torque, the electric power steering apparatus is able to assist steering operation of the driver, thereby reducing loads on the driver.
One example of such electric power steering apparatus is disclosed in Japanese Patent Application Laid-open Publication (JP-A) No. 2010-195142. The disclosed electric power steering apparatus includes a controller that can set an assist torque based on a vehicle speed detected by a vehicle speed sensor, a steering angle detected by a steering angle sensor, and a road surface condition estimated by a road surface condition estimating unit. The steering angle is thus used in the disclosed electric power steering apparatus. Additional to the electric power steering apparatus shown in JP 2010-195142, a sideslip prevention device VSA disclosed in Japanese Patent Application Laid-open Publication (JP-A) No. 2011-162145 also uses the steering angle for operation thereof.
The steering angle means a relative steering angle that represents only a rotational angle of the steering wheel, or an absolute steering angle that represents a rotational angle of the steering wheel and the number of revolutions of the steering wheel. The absolute steering angle can be detected by a mechanical steering angle sensor provided in the vicinity of the steering wheel. The mechanical steering angle sensor may be replaced by a main circuit of an electronic control unit (ECU) disclosed in Japanese Patent Application Laid-open Publication (JP-A) No. 2012-46047, wherein when a key switch is turned on, a resolver detects a rotational angle and the number of revolutions (absolute electric angle) of a brushless motor, and the main circuit calculates an absolute steering angle using the absolute electric angle of the brushless motor and a reduction ratio of a reduction dear.
The ECU disclosed in JP 2012-46047 includes a backup circuit which is configured to detect only the number of revolutions of the brushless motor when the key switch is turned off (namely, when a main power supply of the electric power steering apparatus is turned off). In other words, the resolver should be intermittently excited in order to detect the number of revolutions of the brushless motor even when the steering wheel is operated while the key switch is in the off state.
This means that even when the key switch is in the off state, electric power is necessary for achieving intermittent excitation of the resolver. If the intermittent-exciting period is set to positive infinity to thereby terminate detection of the number of revolutions of the brushless motor by the resolver, the electricity consumption of the backup circuit will be reduced. However, since the resolver is no longer possible to detect a change in the number of revolutions (count-up or count-down) of the brushless motor, when the key switch is switched from the off state to the on state, the main circuit will calculate an erroneous absolute steering angle based on an erroneous number of revolutions detected by the backup circuit.
It is therefore an object of the present invention to provide an electric power steering apparatus which is capable of reducing electricity consumption when a rotational quantity of an electric motor is detected while a main power supply of the electric power steering apparatus is in an off state.